Question

If ΔH = -70.0 kJ and ΔS = -0.500 kJ/K , the reaction is spontaneous below a certain temperature. Calculate that temperature.

Answer 1

We know,

delta G = delta H - T delta S
For a reaction to be spontaneous,
delta G must be negative.

Now since, both delta H and delta S are negative,

The given reaction is not spontaneous for all temperatures.
When delta G is 0, the temperature at that, if lower,
would yield a spontaneous reaction.
0 = delta H - T delta S
-delta H = -T delta S
T = delta H/delta S [feed the given delta H and delta S]
= -70.0kJ/-0.500kJ/K
= 140 K
So any temperature Kelvin lower than 140 K will cause spontaneous reaction